In a known General Packet Radio Service (GPRS) mobile communications network, a Gateway GPRS Support Node (GGSN) acts as a gateway to and from an Internet Service Provider (ISP), and has a connection to a general data communications network. Connected to the GGSN is a Serving GPRS Support Node (SGSN), which is further connected to multiple Base Station Systems (BSS).
A mobile station (MS), which is active within the network, has a connection to one BSS, and the SGSN handles the routing of data from the GGSN to the BSS.
Each BSS has at least one Packet Control Unit (PCU), which handles the different GPRS users, and schedules data on the radio resources which are available for GPRS users in the cell.
When downlink data is transmitted to an MS in a GPRS network, the data is buffered, in the form of Logical Link Control (LLC) Packet Data Units (PDU) both in the SGSN and in the BSS. The LLC is the protocol which provides a logical link between the MS and the SGSN.
There are two modes of LLC operation, as defined in the 3rd Generation Partnership Project Technical Specification 3GPP TS 44.064 V5.0.0 Sections 4.3 and 4.4, namely unacknowledged operation and acknowledged operation.
In acknowledged operation, PDUs are transmitted in numbered Information (I) frames. The SGSN retains a copy of each transmitted LLC-PDU, until this is acknowledged by the MS.
In unacknowledged operation, LLC-PDUs are transmitted in numbered Unconfirmed Information (UI) frames. These UI frames are not acknowledged at the LLC layer, and the SGSN does not need to save LLC-PDUs that are transmitted to the MS.
As further defined in the 3rd Generation Partnership Project Technical Specification 3GPP TS 08.18 V8.9.0, section 8.1, when an SGSN detects a cell change of an MS, the SGSN sends a FLUSH-LL PDU to the BSS controlling the old cell. At that time, the BSS may contain stored PDUs, which are awaiting transmission to the MS. In some cases, for example if the new cell is associated with the same BSS, and if the new cell is in the same routing area as the old cell, it will be possible for those PDUs to be transferred to a queue in the BSS associated with the new cell.
However, if that is not possible, the PDUs are deleted. In that case, this data cannot be transmitted to the mobile station using the LLC protocol. Instead, it must be retransmitted using a higher layer protocol, which can lead to a substantial reduction in the rate at which data can be transmitted to the mobile station.